252 ANNUAL REPORT 
by stakes driven into the lake bottom. This is applicable only where 
deep dredging is practiced, the dredge being moved but slowly. At other 
marl plants the dredged marl is taken by means of scows to the mill and 
pumped from the scow into large reservoirs. The slurry is forced along 
the mains by compressed air; this being far superior to the work of 
plunger pumps. The writer has seen in Germany clays handled by 
stirring up with water in agitators, screening, and then pumping for dis- 
tances as great as two miles. In this way, coarse sand, roots, etc., are at the 
same time removed completely. 
Wherever solid material is being hauled, it should be dumped into 
large bins with steep iron-sheeted bottoms from which the rock can 
be allowed to drop into the chutes leading to the crushers. One man 
can thus feed a large quantity of material with but slight effort. The 
gates for controlling the flow of heavy, coarse material from bins are 
heavy plank affairs, with a few heavy iron teeth which will stop the slid- 
ing of the material with the greatest ease. 
GRINDING OF RAW MATERIALS. 
In the United States we must distinguish clearly three modes of pre- 
paring the raw mixtures, depending on the three combinations of raw 
materials available: 
First, the cement rock and limestone combination employed in the 
Lehigh Valley, Pennsylvania. 
Second, the limestone and clay mixtures wherever suitable limestone 
and clay is found. 
Third, the marl and clay mixtures used in Michigan, Indiana and 
northern Ohio. 
It becomes at once evident that the first and second, working with 
dry and hard material, must employ practically the same class of pre- 
paring machinery, while the materials of the third combination naturally 
call for machinery adapted to working them in the wet state, though, 
of course, this becomes optional to some extent, since the slurry may be 
dried at once, and then worked dry. But this is not the regular pro- 
cedure, especially in working with rotary kilns. 
Preliminary Dryers,—Frequently the clay or the limestone or both 
must be dried before they can be taken through the grinding machines. 
This is accomplished by means of rotary dryers which consist essentially 
of a revolving brick-lined iron tube, 40 feet long and 54 inches in diam- 
eter, revolving on two sets of carrying rollers and driven by a train of 
gearing. Heavy steel tires made in halves are attached to cast iron riding 
rings riveted to the shell. The heat is produced by an ordinary furnace 
fired with coal like a boiler furnace, the heated combustion gases and air 
passing through the tube and out into the stack. The materials to be 
